Introduce term_is_ancestor_of(). Finish taxonomy support for...

Introduce term_is_ancestor_of(). Finish taxonomy support for wp_insert_category(). Props garyc40. fixes #15581

git-svn-id: https://core.svn.wordpress.org/trunk@19678 1a063a9b-81f0-0310-95a4-ce76da25c4cd